1. Field of the Invention
The present invention relates to an organic insulation film used as an insulation film between layers and in particular, to an organic insulation film formation for reducing the wiring parasite capacity.
2. Description of the Related Art
For example, in a GaAs FET (field-effect transistor), it is necessary to reduce the parasite capacity of a gate electrode. For this, as an insulation film covering a T type gate electrode, instead of the conventional Si3N4 (having a specific dielectric constant 7.0) and SiO2 (having a specific dielectric constant 4.0), as shown in FIG. 1, there is a tendency to use an organic insulation film such as benzocyclobutene having a very small specific dielectric constant 2.7.
Moreover, in order to reduce electric interference between wires, such an organic insulation film has begun to be used also for inter-layer insulation in a multi-layered wiring. Moreover, the organic insulation film such as benzocyclobutene has an advantage that a liquid of benzocyclobutene having a low viscosity can easily be applied through rotation and sintered to form a film to obtain a flat surface. Accordingly, this material is also applied to an inter-layer insulation film of a hetero-bipolar transistor as a vertical (non-lateral) device having a number of convex and concave portions.
Conventionally, for etching an organic insulation film, a protection film on the organic insulation film has been used as a mask. For example, in order to obtain an opening (hole) in the benzocyclobutene film, a corresponding hole is made in the protection film made from Si3N4 or SiO2 so as to serve as a mask when subjected to a reactive ion etching using a mixed gas plasma of oxygen and fluorine for dry processing.
However, the conventional organic insulation film formation method has such a problem that if the protection film thickness is increased to obtain a sufficient mask function, the organic insulation film is deteriorated. The organic insulation film should have a small thickness so that sufficient etching can be obtained even with a thin protection film.
For example, a combination of benzocyclobutene as an organic insulation film and Si3N4 or SiO2 as a protection film (serving also as a mask) shows etching ratio as small as 10:1. That is, when the organic insulation film is etched by 10, the protection film is etched by 1. Accordingly, the film thickness of Si3N4 or SiO2 should have a thickness in the order of ⅕ of the benzocyclobutene film.
More specifically, if the benzocyclobutene film has a thickness of 2 micrometers, the Si3N4 or SiO2 film should have a thickness equal to or more than 0.4 micrometers.
On the other hand, when forming a Si3N4 or SiO2 film on the benzocyclobutene, film stress may cause cracks in the benzocyclobutene film. Especially when the Si3N4 or SiO2 film has a thickness equal to or more than 3000 Angstrom, the cracks or other deterioration of benzocylobutene are remarkably increased. For this, the benzocyclobutene has been restricted to about 1 micrometer.
Moreover, the mask materials Si3N4 and the SiO2 are also etched by a significant etching rate and it is difficult to control a remaining film thickness of Si3N4 and SiO2. The remaining film thickness Si3N4 or the SiO2 varies significant, which in turn brings about a problem of irregularity of the specific dielectric constant.
It is therefore an object of the present invention to provide an organic insulation film formation method characterized in the following three points. (1) By reducing the film thickness of the protection film, cracks and other deterioration of the organic insulation film is eliminated. (2) The thickness of the organic insulation film is increased so as to exhibit a sufficient function of the organic insulation film. (3) Irregularities of the thickness of the protection film is reduced.
The present invention provides an organic insulation film formation method comprising: a step of successively forming an organic insulation film, a protection film, and a metal film in this order on a substrate; a step of forming a patterned photo-resist on the metal film; a step of etching the metal film using the photo-resist as a mask; and a step of etching the protection film and the organic insulation film using the remaining metal film as a mask.
In the conventional technique, the protection film is used as a mask and needs to have a sufficient thickness for serving as a mask. In contrast to this, the present invention uses as a mask the metal film on the protection film. Accordingly, the protection film can have a significantly reduced thickness.
In the conventional technique, the organic insulation film should have a film thickness appropriate for the protection film used as a mask. In contrast to this, the organic insulation film can have a thickness regardless of the protection film because the metal film on the protection film is used as a mask. That is, the present invention enables to obtain an organic insulation film having a sufficient thickness.
In the present invention, the metal film is used as a mask for etching the protection film and the organic insulation film. Accordingly, the irregularities of the protection film thickness can be reduced in comparison to the conventional technique in which the protection film is used as a mask for etching the organic insulation film.
When etching the protection film and the organic insulation film using the metal mask as the film, the etching rate of the metal film is preferably equal to or below {fraction (1/10)} of the etching rate of the protection film and the organic insulation film. The organic insulation film is made from, for example, benzocyclobutene (BCB). The protection film is made from, for example, a silicon compound, silicon nitride, silicon oxide, and the like. The metal film is made from, for example, platinum or gold or an alloy containing at least one of platinum and gold.
In other words, the present invention is characterized in that when the organic insulation film is subjected to dry etching, the etching mask is realized by a three-layer configuration constituted by a platinum (Pt) or gold (Au) film, a titanium (Ti) film, and a silicon nitride (Si3N4) or silicon oxide (SiO2) film. This enables to realize a dry etching having a wide selection range and to maintain the film thickness of Si3N4 or SiO2 when formed.
The platinum (Pt) film has a significantly low etching rate for dry etching using a mixed gas plasma of oxygen and fluorine. The etching rate of Pt is smaller than etching rate of the Si3N4 etching rate by several tens and smaller than the etching rate of the organic insulation film etching by several hundreds. Accordingly, the Pt film can serve as an etching mask for the Si3N4 film and the organic insulation film.
Moreover, the Si3N4 film serves as a protection film for the organic insulation film, and is not be etched when making an opening in the organic insulation film. Thus, it is easy to control the final film thickness.
The aforementioned effects can also be obtained when the Pt is replaced by Au, and the Si3N4 is replaced by SiO2.